Biological sciences

June 2011

Biological invasions can have deleterious impacts on native ecosystems, but also offer opportunities for studying evolution. A fundamental question in invasion biology is: what factors are important determining invasion success? Dispersal is essential to the establishment and persistence of populations in heterogeneous environments. This thesis investigated dispersal for its importance in invasions in two ladybirds, Harmonia axyridis (Pallas) and Coccinella septempunctata (L.) (Coleoptera: Coccinellidae). The effects of abiotic and biotic factors on ladybird aerial densities (AD) were investigated using Vertical Looking Radar, a tool which allows for detailed observations of high altitude coccinellid flight for the first time. Temperature was found to be the strongest predictor of AD, wind speed had a negative relationship with AD and aphids had no significant predictive effect. Comparison of H. axyridis distribution and topographical distribution of temperature and wind speed in the U.K. suggests that unfavourable meteorological conditions, for example over the Pennines and Cambrian Mountains, may act as a barrier to dispersal; slowing the spread of the H. axyridis in the U.K. To investigate characteristics of ladybird flight in controlled conditions, a novel in vitro flight test method was designed. This successful method was used to test the hypothesis that increased dispersal ability at range expansion boundaries will be selected for. No significant difference in flight time was found between native and invasive H. axyridis populations. An explanation may be the trade off between dispersal ability and fecundity at range expansion boundaries, which warrants further investigation. The results presented in this thesis add to our understanding of dispersal ability in coccinellids and could be used to help predict future spread of invasive ladybirds. Future work should investigate the potential trade off between dispersal ability and fecundity at range expansion boundaries and whether phenotypic plasticity in dispersal is a predisposition to invasion success.